Lithium nonaqueous batteries have attracted considerable interest in recent years because of their potentially high voltage and high energy content per unit weight. A particularly challenging problem associated with rechargeable lithium nonaqueous batteries is improvement of their recycling characteristics. It would be desirable for many applications that both efficiency and extent of recyclability be improved. It is desirable to accomplish this while maintaining the high voltage and high energy content characteristic of primary lithium batteries.
One of the major problems in nonaqueous cell design is safety, particularly when the cell is exposed to drastic environmental conditions such as electrically shorting the cell, rapid overcharge or rapid discharge. The basic problem is the high reactivity of the cell components (e.g., lithium metal) together with cell design requirements necessary to produce a cell with good capacity, extensive cycle life and reasonable charge/discharge rates.
An important part of cell design for nonaqueous cells is the ingredients and composition of the electrolyte system. The electrolyte system is made up of organic solvent and current-carrying species. Stringent requirements are imposed on solvent and current-carrying species. For example, they must be chemically inert to electrode materials (e.g., lithium metal for the negative electrode, and niobium triselenide for the positive electrode material). It should also exhibit sufficient ionic conductivity so as to permit reasonable charge and discharge rates. Generally, this requires significant solubility of current-carrying species in the solvent. Also, the electrolyte should wet the separator to insure reasonable ionic conductivity through the separator.
Electrolyte systems for nonaqueous batteries have been discussed in a number of references including U.S. Pat. No. 3,928,067, issued to J. Broadhead et al. on Dec. 23, 1975, and U.S. Pat. No. 3,864,167, issued to J. Broadhead et al. on Feb. 4, 1975. The first patent ('067) discloses the use of various polyethylene glycol dialkyl ethers as additives in small amounts (1-5 weight percent) to electrolyte systems for nonaqueous batteries to wet the separator and obtain the high charge and discharge rates. The second patent ('167) describes nonaqueous cells with a number of different positive electrode materials including niobium triselenide.
Particularly desirable is a lithium, nonaqueous, rechargeable cell with high energy density and cycle capacity which is safe, especially when exposed to extreme conditions such as electrical shorting or high temperatures.